Various techniques are known for tuning the wavelength of a semiconductor laser.
A common approach for tuning the wavelength of a semiconductor laser is based on current injection into the optical cavity. However, this technique does not provide a wide tuning range due to the limitation on the maximum refractive index change that can be induced by current injection. In addition, current tuning is accompanied by large changes in the optical output power, which is often not desirable.
In order to enhance the tuning range of a semiconductor laser without large changes in the optical output power, it has been proposed to use passive microring resonators outside the laser active region.
Microring resonators are known for their small size, high quality factor Q, transparency to off-resonance light, and absence of intrinsic reflections.
A first tunable laser structure using microring resonators has been proposed in the article by S. Matsuo et al. entitled “Microring-resonator-based widely tunable lasers” published in IEEE Journal of Selected Topics in Quantum Electronics, vol. 15, No. 3, May/June 2009, pp. 545-554, which discloses a double-ring resonator tunable laser (DDR-TL). This structure comprises an active region and a filter region. The active region is in the center of the device and provides the optical gain. The filter region comprises two passive microrings which are coupled to the gain region of the laser. The two microrings act as filters, having each a comb-like frequency response with different free spectral ranges (FSRs). Only light at the common resonant frequency of both microrings can be amplified. The lasing frequency can be selected by controlling the resonant frequency of the ring resonators according to the Vernier effect.
More recently, a second tunable laser structure using microring resonators has been proposed in the article by Po Dong et al. entitled “Silicon photonic devices and integrated circuits” published in Nanophotonics 2014, Vol. 3, No. 4-5, pp. 215-228.
The latter laser structure, 100, is schematically represented in FIG. 1. It comprises a III-V semiconductor gain section (for example made of InP-based materials), 110, integrated and coupled with a first silicon waveguide, 120, having a first end, 121, and a second end, 122.
A first microring resonator in silicon, 130, is arranged between the first end 121 and the gain section 110. The microring resonator 130 is evanescently coupled, 123, with the first silicon waveguide 120 and with a second silicon waveguide, 140, arranged in parallel with the first silicon waveguide.
Similarly, a second microring resonator in silicon, 150, is arranged between the gain section 110 and the second end 122 of the first silicon waveguide 120. The second microring resonator is also evanescently coupled, 124, with the first silicon waveguide and with a third silicon waveguide, 160, arranged in parallel with the first silicon waveguide.
The second silicon waveguide 140 comprises a proximal end, 141, provided with a first Bragg reflector, and a distal open end, 142. Similarly, the third silicon waveguide 160 comprises a proximal end, 161, provided with a second Bragg reflector and a distal open end, 162.
The light output leftwards from the gain section 110 is partially injected via evanescent coupler 123 into the first microring resonator 130 to give rise to a first clockwise propagating wave. This wave is then coupled into the second silicon waveguide 140 to give rise to a rightwards propagating wave. The rightwards propagating wave is reflected by the first Bragg reflector 141 back into the first microring resonator 130, where it gives rise to a first counterclockwise propagating wave.
Similarly, the light output rightwards from the gain section 110 is partially injected by evanescent coupling 124 into the second microring resonator 150 to give rise to a second counterclockwise propagating wave. This wave is then coupled into the third silicon waveguide 160 to give rise to a leftward-propagating wave. The leftward-propagating wave is reflected by the second Bragg reflector 161 back into the second microring resonator 150, where it gives rise to a second clockwise propagating wave.
The second end 122 of the first silicon waveguide is provided with a Bragg grating to output light vertically e.g. into an optical fiber (not shown), using for example the approach described by G. Roelkens et al. in the article entitled “Grating-based optical fiber interfaces for silicon-on-insulator photonic integrated circuits” published in IEEE Journal of Selected Topics in Quantum Electronics 2011, Vol. 17, No. 3, pp. 571-580.
The first and second microring resonators have slightly different radii and thus exhibit comb-like frequency responses with slightly different FSRs. Heaters, 180, 190, respectively provided on the first and the second microring resonators, slightly modify the resonant frequencies due to temperature dependence of the refractive index. The Vernier effect between the two frequency combs can be used for tuning the wavelength of the laser, the wavelength being selected by the overlap between the two frequency combs.
The silicon waveguides are typically obtained by patterning an SOI wafer. The InP-based gain section is obtained by growing a stack of epitaxial layers on an InP wafer and by bonding the unprocessed InP wafer, epitaxial layers down, onto the patterned SOI wafer. After bonding, the InP substrate is removed and the diode lasers can be fabricated using conventional wafer-scale processing, and lithographically aligned to the underlying SOI pattern.
However, the laser structure shown on FIG. 1 is unstable under normal operating conditions. In particular, experiments have demonstrated that the laser structure was prone to mode jumps and chaotic oscillations. Furthermore, the efficiency of the laser is rather poor and therefore the light output power quite low.
The purpose of the present application is therefore to provide a wavelength tunable semiconductor laser remedying the shortcomings recited above. In particular, the main object of the present invention is to propose a wavelength tunable semiconductor laser structure which exhibits a very high degree of stability and high efficiency.